The accurate determination of formation fluid saturation (e.g., hydrocarbon or water saturation) is a useful step in the petrophysical evaluation of conventional reservoirs. Commonly, open-hole resistivity-based models together with porosity logs are used for water saturation estimations owing to their availability, robustness and deep depth of investigation. To be accurate, however, these models utilize knowledge of formation water salinity (i.e., formation water resistivity) and formation properties (e.g., tortuosity factor, cementation exponent, corrections for matrix conductivity), and these models may fail in environments with low or unknown salinity, or with abundant and varying clay content. A quantitative estimation of remaining hydrocarbon saturation in reservoirs under enhanced oil recovery, such as water or steam flood, is particularly challenging using resistivity-based models because of differences in salinity of formation and injection waters and the effect of imbibitions on the saturation exponent, yet is useful for assessing hydrocarbon sweep efficiency and predicting future hydrocarbon production (Al-Harbi et al., 2010). Incorrect log analyst assignments in resistivity-based models can lead to significantly erroneous determinations of fluid saturations.
Formation sigma and inelastic carbon/oxygen logging are two common techniques for water or hydrocarbon saturation determination in cased-hole environments. Formation sigma is sensitive primarily to chloride in water. Similar to resistivity-based logging, this approach utilizes knowledge of formation salinity in order to estimate water volumes and has poor sensitivity in reservoirs with fresh waters. It also uses knowledge of matrix sigma. Salinity-independent inelastic carbon/oxygen logging is often used for saturation evaluation behind casing in environments with low or unknown salinity. In this technique, gamma ray energy spectra are decomposed into net inelastic yields from C and O, and capture yields from matrix elements (e.g., Si, Ca, Fe). A lithology-dependent model is used to partition C and O yields into respective contributions from the rock matrix, pore fluids and borehole fluids and the model data are inverted to obtain an estimate of oil volume. Interpretation of carbon/oxygen logs is complex, using extensive calibration and knowledge of the formation lithology, because oxygen is ubiquitous in the rock matrix and in formation and borehole fluids, and carbon is commonly present both in organic and inorganic formation components. The log adjustment for rock matrix contributions and uncertainties of the derived formation fluid carbon/oxygen ratio increase significantly at low porosities, so this method is typically applied in formations with porosities greater than 15 p.u.